This present invention relates to a method for making a doped well in a semiconductor substrate.
In the manufacture of semiconductor devices it is frequently necessary to make a doped well in a semiconductor substrate. For example, in the manufacture of CMOS devices p-channel devices must be formed in n-type semiconductor and n-channel devices must be formed in p-type semiconductor. Thus, if CMOS devices are formed on an n-type substrate it is necessary to make p-type wells in the n-type substrate and form the n-channel devices within the p-type wells.
Conventionally, such doped wells are formed by growing a thick oxide on a semiconductor substrate, etching an opening through the thick oxide at a location where a doped well is desired, growing a thin oxide in the opening, implanting a dopant through the thin oxide, and heating in an oxidizing ambient to drive the dopant into the substrate.
The thick oxide absorbs dopant to prevent implantation in regions of the substrate where no dopant is desired. The thin oxide is universally used to reduce ion channeling effects which are generally believed to cause unacceptable deviations from the desired doping profile for the well. The drive-in step is done in an oxidizing ambient to thicken the thin oxide over the well. When this thickened oxide is removed, it leaves a recess in the substrate over the doped well. This recess is a visible feature which is used to locate the well for mask alignment in later processing steps.